In The Silmarillion and this post, Buzz mentioned that "Fëanor will finally repent his crimes and be released from the Halls of Mandos" so that the two trees can be resurrected once more.
What were Fëanor's crimes?
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Feanor's crimes were many.
First, he spoke rebellious words against the Valar.
Thus with lies and evil whisperings and false counsel Melkor kindled the hearts of the Noldor to strife; and of their quarrels came at length the end of the high days of Valinor and the evening of its ancient glory. For Fëanor now began openly to speak words of rebellion against the Valar, crying aloud that he would depart from Valinor back to the world without, and would deliver the Noldor from thraldom, if they would follow him. (The Silmarillion)
He then drew a sword in anger against his brother. This was the reason for his first banishment.
But even as Fingolfin spoke, Fëanor strode into the chamber, and he was fully armed: his high helm upon his head, and at his side a mighty sword. ‘So it is, even as I guessed,’ he said. ‘My half-brother would be before me with my father, in this as in all other matters.’ Then turning upon Fingolfin he drew his sword, crying: ‘Get thee gone, and take thy due place!’
Fingolfin bowed before Finwë, and without word or glance to Fëanor he went from the chamber. But Fëanor followed him, and at the door of the king's house he stayed him; and the point of his bright sword he set against Fingolfin's breast. 'See, half-brother!’ he said. This is sharper than thy tongue. Try but once more to usurp my place and the love of my father, and maybe it will rid the Noldor of one who seeks to be the master of thralls.’
[...] And [Mandos said] this deed was unlawful, whether in Aman or not in Aman. Therefore this doom is now made: for twelve years thou shalt leave Tirion where this threat was uttered. (Silmarillion)
He did suffer his sentence for this, and was released by his brother, but he didn't exactly repent. In fact, when Morgoth attacked the Trees, he violated his decree of banishment and rebelled against the Valar not just in word, but deed.
Then suddenly Fëanor appeared in the city and called on all to come to the high court of the King upon the summit of Túna; but the doom of banishment that had been laid upon him was not yet lifted, and he rebelled against the Valar. (The Silmarillion)
He then swore an evil oath, that was the cause of very much later evil:
Then Fëanor swore a terrible oath. His seven sons leapt straightway to his side and took the selfsame vow together, and red as blood shone their drawn swords in the glare of the torches. They swore an oath which none shall break, and none should take, by the name even of Ilúvatar, calling the Everlasting Dark upon them if they kept it not; and Manwë they named in witness, and Varda, and the hallowed mountain of Taniquetil, vowing to pursue with vengeance and hatred to the ends of the World Vala, Demon, Elf or Man as yet unborn, or any creature, great or small, good or evil, that time should bring forth unto the end of days, whoso should hold or take or keep a Silmaril from their possession. (The Silmarillion)
But thou Fëanor Finwë's son, by thine oath art exiled. The lies of Melkor thou shalt unlearn in bitterness. (The Silmarillion)
Then, drawing many of the other Noldor in rebellion with him, he instigated the Kinslaying and murdered many of the Teleri for their ships.
Thereupon Fëanor left him, and sat in dark thought beyond the walls of Alqualondë, until his host was assembled. When he judged that his strength was enough, he went to the Haven of the Swans and began to man the ships that were anchored there and to take them away by force. But the Teleri withstood him, and cast many of the Noldor into the sea. Then swords were drawn, and a bitter fight was fought upon the ships, and about the lamplit quays and piers of the Haven, and even upon the great arch of its gate. (The Silmarillion)
This led to the Doom of the Noldor, the harsh judgement of the Noldor by the Valar. But Feanor continued on foward, and later, when it came to the sea-crossing, betrayed many of the Noldor, and burned the ships.
Therefore it came into the hearts of Fëanor and his sons to seize all the ships and depart suddenly; for they had retained the mastery of the fleet since the battle of the Haven, and it was manned only by those who had fought there and were bound to Fëanor. And as though it came at his call, there sprang up a wind from the north-west, and Fëanor slipped away secretly with all whom he deemed true to him, and went aboard, and put out to sea, and left Fingolfin in Araman. And since the sea was there narrow, steering east and somewhat south he passed over without loss, and first of all the Noldor set foot once more upon the shores of Middle-earth; and the landing of Fëanor was at the mouth of the firth which was called Drengist and ran into Dor-lómin.
But when they were landed, Maedhros the eldest of his sons, and on a time the friend of Fingon ere Mor-goth's lies came between, spoke to Fëanor, saying: 'Now what ships and rowers will you spare to return, and whom shall they bear hither first? Fingon the valiant?’
Then Fëanor laughed as one fey, and he cried: ‘None and none! What I have left behind I count now no loss; needless baggage on the road it has proved. Let those that cursed my name, curse me still, and whine their way back to the cages of the Valar! Let the ships burn!’ Then Maedhros alone stood aside, but Fëanor caused fire to be set to the white ships of the Teleri. So in that place which was called Losgar at the outlet of the Firth of Drengist ended the fairest vessels that ever sailed the sea, in a great burning, bright and terrible. And Fingolfin and his people saw the light afar off, red beneath the clouds; and they knew that they were betrayed. This was the firstfruits of the Kinslaying and the Doom of the Noldor. (The Silmarillion)